Studies of Fluid Flow Indicators, Pacific Margin of Costa Rica

Eli Silver, Brian McAdoo, Marcus Langseth, and Daniel Orange

Seismic reflection profiles off Costa Rica image a decrease in thickness of the underthrust sedimentary section from the Middle America Trench, implying a significant reduction of porosity in the outer 3-5 km from the trench and a source of vent water through the wedge. We encountered no evidence of discrete fluid venting over the outer 3-5 km of this margin from dives using the ALVIN submersible or from heat flow measurements (based on absence of chemosynthetic vent communities and heat flow anomalies in this zone). Vent communities occur farther upslope, associated with a series of out-of-sequence thrusts, with two mud diapirs, and a mid-slope canyon. We infer that fracture permeability dominates in the out-of-sequence thrusts, upflow of fluid-rich muds in the diapir, a d focusing of fluid flow in the canyon. Over 100 heat flow observations on the wedge and incoming COCOS plate showed a broad area of anomalously low heat flow (13 mW/m²) seaward of the frontal thrust, whereas the expected heat flow for ocean crust of early Miocene age is seven times greater. The very low regional heat flow may reflect refrigeration by vigorous sea water flow through the upper crust pillow basalts. Heat flow increases to about 30 mW/m² throughout the lower slope to mid-slope, implying a combination of widespread fluid venting, reheating of the cooled crust and frictional heating at the base of the wedge. The lack of discrete vents over the outer 3-5 km of the margin indicates diffuse flow and likely temporal episodicity, as this region has been aseis ic since 1950.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California